Acorn masting has been believed to cause positive responses in rodent population dynamics, but a considerable number of studies show that it is not always the case. Acorns are generally nutritious, but acorns of some species contain a high level of tannins, which cause negative effects on consumers. In this study we tested the hypothesis that rodent population responses to acorn masting may vary depending on differences in tannin tolerance among rodent species, using three sympatric rodent species in Hokkaido, Japan. The effects of acorn abundance on rodent population densities were analyzed using the dataset obtained in the Uryu Experimental Forest of Hokkaido University, in which population fluctuations of the three rodent species (Apodemus speciosus, A. argenteus, and Myodes rufocanus) and crop of Quercus crispula acorns have been monitored since 1992. Population growth rates of A. speciosus were higher in the next year after mast years than in those after non-mast years, but this tendency was not observed in the other species. To compare tannin tolerance, an acorn feeding experiment was conducted. Test animals were fed only Quercus crispula acorns for 6 d after 2 wk of tannin-acclimation period. A.speciosus increased their body weight during this period, whereas A. argenteus and M. rufocanus lost 6.8% and 13.6% of their weight, respectively. This indicates that A. speciosus has higher tannin tolerance. These differences in tannin tolerance were consistent with population responses to acorn masting. This finding emphasizes the importance of evaluating tannin tolerance, to understand rodent population dynamics.